This invention relates to an electrolytic capacitor and more particularly to a non-aqueous electrolyte for an electrolytic capacitor, especially an aluminum electrolytic capacitor.
As is well-known, electrolytic capacitors comprise an anode of a so-called valve metal which has an oxide film on it in contact with an electrolyte. Two of the most commonly used metals are aluminum and tantalum. Aluminum is widely used because of its lower density, lower cost, availability in high-purity form, and relative ease of reformation.
Many electrolytes for aluminum capacitors have unsatisfactory low- and/or high- temperature characteristics. Thus, aqueous electrolytes are restricted generally to operation above the freezing-point and below the boiling-point of the solvent, i.e., water. While ethylene glycol has a lower freezing-point and a higher boiling-point than water, its resistivity at low temperatures is unsatisfactory. N,N-dimethylformamide, another preferred solvent, has a low freezing-point but a boiling-point below that of ethylene glycol.
Beside having a low-freezing and a high-boiling point, an electrolyte solvent should have a high dielectric constant and be chemically neutral to the electrodes and the dielectric oxide. Such solvents include 4-butyrolactone, 3-methoxypropionitrile, propylene carbonate, methyl carbitol, and butyl cellosolve.
Unfortunately, many solutes which have been used in the past with non-aqueous solvents, especially dimethylformamide, are not necessarily useful with other non-aqueous solvents, e.g., 4-butyrolactone.